Write essay on the evolution of wheat, include diagrams and information on the Ph1 locus
(max 1000 words, excluding references). For completion of first draft Monday of week 10
The Evolution of Wheat, including Diagrams and Information on the Ph1 locus Triticum aestivum, more commonly known as bread wheat, is an allohexaploid which forms 21 pairs of chromosomes during meiosis. An allopolyploid organism has two or more sets of homologous chromosomes obtained from different species. An allohexaploid organism such as T. aestivum contain seven sets of chromosomes, two sets from three genomes A, B and D whose respective origins are Triticum uratu, Aegilops speltoides and Triticum tauschii. Each genome normally contains sevenpairs of chromosomes as seen in figures 1. & 2. Therefore T. aestivum is AABBDD, with 2n = 42. Having more than one set of chromosomes means that the organism can select from a larger range of alleles and hence there can be a wider range of phenotypic and genotypic characters . The disadvantage of being polyploidy is that there is twice as much DNA to copy Note: twice or more if the ploidy levels are higher., therefore there are more chances of mistakes being made in meiosis. However, the presence of extra copies of the same genes increases the organisms’ robustness.
(Figure 1)
(Figure 2)
Figure 1. A karyotype of genomes A,B and D. Figure 2. An ideogramme mapping the genes on each chromosome, the functions of which are summarised in the table below.
Table of mapped genes in figure 2. and their functions.
Gene
Function
Ph1 & Ph2
Regulating polyploid meiosis, note the position of Ph1 on chromosome 5B.
R
Grain colour.
Ha
Texture.
Wx
Starch composition; different varieties of wheat contain different starches.
Ppd
Photoperiod gene reacts with day length to control yield.
Vrn
Cooling of seeds during germination, to accelerate the rate of flowering (vernalization), makes wheat one of the most widely adapted plant.
Rf
Restorer gene restores male fertility.
Ms & ms
Male sterility.
Gli & Glu
Gluten.
Sr2 & Lr34
Durable diseases resistance.
Qfhs
QTL; quantitative trait locus.
Tg, Q, s1, C, Rht, Hd, B1 & B2
Domestication.
Triticum aestivum arose as a result of a two step sexual hybridization process. The initial step involved the hybridization of the A genome progenitor, identified as Triticum uratu, and the B genome progenitor, Aegilops speltoides, to form a primitive tetraploid wheat known as Triticum durum (more commonly known as pasta wheat), with 2n=14, AABB. In the second step, this tetraploid (T. durum), then hybridizes with the D genome progenitor, known as Triticum tauschii (a wild species of goat grass consisting of 14 chromosomes) to form the basic hexaploid configuration, AABBDD. Both events occur in the B genome cytoplasm. The accuracy and efficiency involved in this two step hybridization, (see figure 3.), has an important influence on the fertility of the wheat plant which is crucial for successful breeding.
(Figure 3. annotated image from New Hall Mill: The Evolution of Wheat)
Despite having more than two sets of chromosomes, T. durum and T. aestivum act as diploids during meiosis, with homologous chromosomes pairing, e.g. In T. aestivum, chromosome 1B pairs with 1B, and not with homoeologous chromosomes 1A, or 1D. The three genomes A, B and D are quite similar and hence it is important that during meiosis, the homologous chromosomes are paired, and not the homoeologous chromosomes. Homoeologous chromosomes are partly homologous; hence have some of the same genetic material in the same order, but different sequences of repetitive DNA.
The Ph1 locus, which is found on chromosome 5B controls the pairing of homologous chromosomes. It is defined at a region which contains the Cdk (cyclin dependent kinase) related complex. Cdk’s are a group of evolutionarily conserved protein kinases which are involved in regulating the cell cycle. The Ph1 locus is mainly involved in controlling chromosome recognition and how chromosomes pair in meiosis. For pairing and recombination to occur, the wheat chromosomes undergo a process called chromatin remodelling at the start of meiosis, which is essential for chromosome segregation. This process involves changes in structure of the chromatin complex. The presence of the Ph1 gene enforces bivalent pairing in polyploid wheat as chromosomes which come into contact with each other must be homologous for chromatin remodelling to occur. It thus restricts the formation of chiasma between homoeologous chromosomes and different chromosomes e.g. the pairing of 5A and 1D. This mechanism prevents the interbreeding of wheat species to generate desirable characteristics, therefore wheat breeders will often knock this gene out. This supports the fact that the Ph1 gene is able to halt the recombination between the wheat and related chromosomes (Colas et al., 2008).
Ph1 acts upon the chromosomes’ centromeres. During meiosis, chromosome telomeres pack together into a bouquet formation and when Ph1 is present, the chromosomes are arranged in pairs on the nuclear membrane via their centromeres, whereas when Ph1 is absent, the chromosomes remain unpaired. However, according to Griffiths et al. (2008), the chromosomes are still able to pair correctly via their telomeres, regardless of whether Ph1 is present or not. Note: the same problem, is Ph1 involved then? When pairing occurs, a paired structure is formed with telomeres at its apex. The chromosomes then attach and are “zipped up” unless the chromosomes are in different structural arrangements. In which case, they will not be “zipped”, but instead, are held together by pegging, to give a less organised looped structure (see figure 4. below).
This suggests that although Ph1 is not vital for chromosome pairing, it promotes efficient meiosis.
(Figure 4. The Moore Lab: Wheat meiosis and the Ph1 locus .)
The presence of the Ph1 gene enabled the speciation of wheat to produce the vastly hybridised polyploid Tristicum aevistium. It allowed the polyploid hybrid to divide as a diploid organism and facillitated the even distribution of all sets of chromosomes, resulting in fertile offspring of this hybrid species and ensuring that all progeny have the incured advantages of all 3 chromosome sets. Although Ph1 is a hinderance when breeding for new species of wheat, it is essential for organised meiosis in exhisting species. An ideal situation would be one where the Ph1 gene could be switched on and off at different stages of meiosis to produce advantageous results.
word count: 982
References Yousafzai F.K., Al-Kaff. N, Moore.G. (2010). Structural and functional relationship between the Ph1 locus protein 5B2 in wheat and CDK2 in mammals. Functional and integrative Genomics. 10(2), 157-166.
Lukaszewski, A.J. and Kopecky, D.(2010). The Ph1 locus from wheat controls meiotic chromosome pairing in autotetraploid rye (Secale cereale L.). Cytogenetic and genome research. 129(1-3), 117-123.
Griffiths, A.J.F, Wessler, S.R., Lewontin, R.C. and Carroll, S.B. (2008). Introduction to genetic analysis.( 9th ed). W.H. Freeman Publishing.
Colas, I., Shaw, P., Prieto, P., Wanous, M., Spielmeyer, W. Mago, R. and Moore, G. (2008). Effective chromosome pairing requires chromatin remodelling at the onset of meiosis. Proceedings of the national academy of sciences of the USA. 105(16), 6075-6080.
Athwal RS and Kimber G. (1972). A Reassment of the Course of Evolution of Wheat. Proc Natl Acad Sci U S A., 69(4): 912-915
Basu A., Basu D., Chakraborti P., Chakraborty A., Das S., Maiti MK. and Sen SK. 2010. An unedited 1.1 kb mitochondrial orfB gene transcript in the Wild Abortive Cytoplasmic Male Sterility (WA-CMS) system of Oryza sativa L. subsp. indica. BMC Plant Biology. 10(39)
Group secretary plus email:Etomby Namme- bt09066@qmul.ac.uk
Group members plus email:
Marium Sherwani - bt09076@qmul.ac.uk
Samaah Haque - bt09089@qmul.ac.uk
Shaima Al-Zawawi - bt09083@qmul.ac.uk
Sabrina Brown- bt09061@qmul.ac.uk
The Evolution of Wheat, including Diagrams and Information on the Ph1 locusTriticum aestivum, more commonly known as bread wheat, is an allohexaploid which forms 21 pairs of chromosomes during meiosis. An allopolyploid organism has two or more sets of homologous chromosomes obtained from different species. An allohexaploid organism such as T. aestivum contain seven sets of chromosomes, two sets from three genomes A, B and D whose respective origins are Triticum uratu, Aegilops speltoides and Triticum tauschii. Each genome normally contains seven pairs of chromosomes as seen in figures 1. & 2. Therefore T. aestivum is AABBDD, with 2n = 42. Having more than one set of chromosomes means that the organism can select from a larger range of alleles and hence there can be a wider range of phenotypic and genotypic characters . The disadvantage of being polyploidy is that there is twice as much DNA to copy Note: twice or more if the ploidy levels are higher., therefore there are more chances of mistakes being made in meiosis. However, the presence of extra copies of the same genes increases the organisms’ robustness.
Table of mapped genes in figure 2. and their functions.
Despite having more than two sets of chromosomes, T. durum and T. aestivum act as diploids during meiosis, with homologous chromosomes pairing, e.g. In T. aestivum, chromosome 1B pairs with 1B, and not with homoeologous chromosomes 1A, or 1D. The three genomes A, B and D are quite similar and hence it is important that during meiosis, the homologous chromosomes are paired, and not the homoeologous chromosomes. Homoeologous chromosomes are partly homologous; hence have some of the same genetic material in the same order, but different sequences of repetitive DNA.
The Ph1 locus, which is found on chromosome 5B controls the pairing of homologous chromosomes. It is defined at a region which contains the Cdk (cyclin dependent kinase) related complex. Cdk’s are a group of evolutionarily conserved protein kinases which are involved in regulating the cell cycle. The Ph1 locus is mainly involved in controlling chromosome recognition and how chromosomes pair in meiosis. For pairing and recombination to occur, the wheat chromosomes undergo a process called chromatin remodelling at the start of meiosis, which is essential for chromosome segregation. This process involves changes in structure of the chromatin complex. The presence of the Ph1 gene enforces bivalent pairing in polyploid wheat as chromosomes which come into contact with each other must be homologous for chromatin remodelling to occur. It thus restricts the formation of chiasma between homoeologous chromosomes and different chromosomes e.g. the pairing of 5A and 1D. This mechanism prevents the interbreeding of wheat species to generate desirable characteristics, therefore wheat breeders will often knock this gene out. This supports the fact that the Ph1 gene is able to halt the recombination between the wheat and related chromosomes (Colas et al., 2008).
Ph1 acts upon the chromosomes’ centromeres. During meiosis, chromosome telomeres pack together into a bouquet formation and when Ph1 is present, the chromosomes are arranged in pairs on the nuclear membrane via their centromeres, whereas when Ph1 is absent, the chromosomes remain unpaired. However, according to Griffiths et al. (2008), the chromosomes are still able to pair correctly via their telomeres, regardless of whether Ph1 is present or not. Note: the same problem, is Ph1 involved then? When pairing occurs, a paired structure is formed with telomeres at its apex. The chromosomes then attach and are “zipped up” unless the chromosomes are in different structural arrangements. In which case, they will not be “zipped”, but instead, are held together by pegging, to give a less organised looped structure (see figure 4. below).
This suggests that although Ph1 is not vital for chromosome pairing, it promotes efficient meiosis.
The presence of the Ph1 gene enabled the speciation of wheat to produce the vastly hybridised polyploid Tristicum aevistium. It allowed the polyploid hybrid to divide as a diploid organism and facillitated the even distribution of all sets of chromosomes, resulting in fertile offspring of this hybrid species and ensuring that all progeny have the incured advantages of all 3 chromosome sets. Although Ph1 is a hinderance when breeding for new species of wheat, it is essential for organised meiosis in exhisting species. An ideal situation would be one where the Ph1 gene could be switched on and off at different stages of meiosis to produce advantageous results.
word count: 982
References
Yousafzai F.K., Al-Kaff. N, Moore.G. (2010). Structural and functional relationship between the Ph1 locus protein 5B2 in wheat and CDK2 in mammals. Functional and integrative Genomics. 10(2), 157-166.
Lukaszewski, A.J. and Kopecky, D.(2010). The Ph1 locus from wheat controls meiotic chromosome pairing in autotetraploid rye (Secale cereale L.). Cytogenetic and genome research. 129(1-3), 117-123.
Griffiths, A.J.F, Wessler, S.R., Lewontin, R.C. and Carroll, S.B. (2008). Introduction to genetic analysis.( 9th ed). W.H. Freeman Publishing.
Colas, I., Shaw, P., Prieto, P., Wanous, M., Spielmeyer, W. Mago, R. and Moore, G. (2008). Effective chromosome pairing requires chromatin remodelling at the onset of meiosis. Proceedings of the national academy of sciences of the USA. 105(16), 6075-6080.
Athwal RS and Kimber G. (1972). A Reassment of the Course of Evolution of Wheat. Proc Natl Acad Sci U S A., 69(4): 912-915
Basu A., Basu D., Chakraborti P., Chakraborty A., Das S., Maiti MK. and Sen SK. 2010. An unedited 1.1 kb mitochondrial orfB gene transcript in the Wild Abortive Cytoplasmic Male Sterility (WA-CMS) system of Oryza sativa L. subsp. indica. BMC Plant Biology. 10(39)
Australian Government Department of Health and Ageing, Office of the Gene Technology Regulator: The Biology of Triticum aestivum L. em Thell. (Bread Wheat), Version 2: February 2008. Avaliable at:
http://www.health.gov.au/internet/ogtr/publishing.nsf/Content/wheat-3/$FILE/biologywheat08.pdf
Genetics: A publication of The Genetics Society of America. Image avaliable at:
http://www.genetics.org/cgi/content/full/168/2/1087/FIG3
The Moore Lab: Wheat meiosis and the Ph1 locus. Availiable at:
http://www.jic.ac.uk/staff/graham-moore/wheat_meiosis.htm
New Hall Mill: The Evolution of Wheat - Introduction. Available at:
http://www.newhallmill.org.uk/wht-evol.htm
Student IDs
Shaima Al Zawawi: 090137240 Sabrina Brown:090128259
Samaah Haque: 090141269 Etomby Namme: 090129407
Marium Sherwani: 090134663
Note..very good....not enough was done to improve the grade